Crankshaft machining and finishing

ABSTRACT

A method of machining rotationally symmetrical parts, in particular crankshafts, in particular the bearing surfaces of crankshafts, until they are in a condition of being ready for use, that is to say the condition in which the crankshaft can be installed in an engine without the further removal of material at the bearing surfaces. The object of the invention is to simplify the removal of material when machining bearing locations of a crankshaft. In the method according to the invention for finishing machining rotary parts, in a condition such that they are ready for use, after the original shaping operation removal of material is effected only by cutting machining with a given cutting edge and subsequent finishing.

I. FIELD OF USE

The invention concerns a method of machining rotationally symmetricalparts, in particular crankshafts, in particular the bearing surfaces(both of the big-end bearings and also the central or main bearings) ofcrankshafts to put them into the condition in which they are capable ofuse, that is to say the condition in which the crankshaft can be fittedin the engine without further removal of material at the bearingsurfaces.

In a practical context, the percentage contact area is ascertained by aprocedure which comprises pressing against the surface to be determined,a counterpart surface of ideal shape, that is to say when dealing withflat surfaces, an ideally flat surface or, in the present case, whendealing with external round surface, a concave counterpart surface whichideally corresponds to a circular arc, under a given nominal loading,for example 0.1 N/mm². By virtue of that nominal loading, themicroscopic raised portions of the profile which without a loading wouldonly bear against the counterpart surface with their tips and thus witha surface proportion of tending closely towards 0 are pressed somewhatflat so that the contacting surface proportion rises with respect to thetotal surface area and in practice can be satisfactorily ascertained bydyeing or tinting and so forth. At the given nominal loading mentioned,the percentage contact area is between 20% and 40% at a transfer betweencutting machining and finishing operations. This same percentage contactarea is less than 50% of the percentage contact area occurring after thefinishing operation.

II. TECHNICAL BACKGROUND

Crankshafts, in particular the crankshafts for private motor vehicleswhich have a large number of cylinders are known to be workpieces whichare unstable during machining and thus difficult to machine. Assessmentof the dimensional accuracy of a finished crankshaft is effectedprimarily, besides the axial bearing width, by assessment of thefollowing parameters:

Diameter deviation=deviation from the predetermined reference or targetdiameter of the bearing journal,

roundness=macroscopic deviation from the round reference or targetcontour of the bearing journal,

concentricity=diameter deviation in the case of a rotating workpiece,that is to say for example the deviation from the reference positionalcontour which a main bearing location effects during the rotary movementof the crankshaft by virtue on the one hand of the non-ideal roundnessof that main bearing Journal and on the other hand by virtue of theout-of-center journal of the crankshaft which in that case is supportedonly at its ends,

roughness R_(a)=a value which is ascertained by calculation and whichrepresents the microscopic roughness of the surface of the bearinglocation, and

percentage contact area=the load-bearing surface proportion of thesurface structure, considered microscopically, which comes into contactwith a co-operating or counterpart surface pressed thereon, and inaddition, in regard to the big-end bearing locations,

stroke deviation=dimensional, percentage deviation of the actual stroke(spacing of the actual center of the big-end bearing journal from theactual center of the main bearings), from the reference or targetstroke, and

angle deviation=deviation of the actual angular position of the big-endbearing journal from its reference or target angular position relativeto the main bearing axis and with respect to the angular position inrelation to the other big-end bearing journals, the angle deviationbeing specified in degrees or as a longitudinal dimension in theperipheral direction, related to the stroke,

wherein observing the desired tolerances in regard to those parametersis made difficult less due to the available machining methods than theinstability of the workpiece and the machining forces involved. Theefficiency and economy of the method also play a large part in apractical context.

Hitherto the removal of material from the bearing locations on thecrankshaft in its original form, that is to say as cast or forged, waseffected in succession in three machining steps:

First Step:

Cutting machining with a given cutting edge; this involved using theprocesses of turning, rotary broaching, turning-rotary broaching,internal round milling and external milling, rotary milling, inparticular in the form of high-speed milling or combinations of suchprocedures. The magnitude of the material to be removed was in themillimeter range.

Second Step:

Grinding by means of a hard, massive grinding tool, for example agrinding wheel, which generally rotates with its axis of rotation inparallel relationship with the axis of rotation of the crankshaft to bemachined; the amount of material to be removed was in thetenths-of-millimeter range.

In the case of crankshafts which are difficult to machine, in particularcrankshafts which are long and thus highly unstable, the grindingmachining operation was also effected in a multi-stage procedure, forexample in a two-stage procedure by preliminary and finishing grinding.

Third Step:

Finishing by generally a stationary grinding means (grinding belt orgrinding stone) which is pressed against the external periphery of therotating bearing location; the amount of material to be removed is atthe present time in the range of hundredths of a millimeter or even μm.

In that respect, a distinction is also to be drawn in regard to themachining operation, in respect of the material of the crankshaft (steelor cast, iron), in which connection in particular steel crankshaftswhich are preferably used for situations of use involving a high loadingare hardened at the surfaces of the bearing locations, after the cuttingmachining operation. That gives rise to renewed distortion of thecrankshaft, and such distortion had to be compensated by grinding andfinishing. Hardening of cast iron crankshafts is at the present timealready omitted in many cases and can be completely avoided by using acast iron material of relatively great hardness, for example GGG 60 or70 or more and improved strength values.

In order to reduce the costs involved in crankshaft machining, theendeavour is to reduce the machining of the bearing locations from threeto two different machining stages.

This means however that in particular the removal of material which isto be implemented by the grinding operation must be greater than in thecase of a three-stage method. Removing material by means of grindinghowever involves the following disadvantages:

because of the cooling/lubricating agent which is to be added, thegrinding slurry which is produced gives rise to problems and isextremely costly to dispose of,

because of the oil contained in the cooling/lubricating agent, forexample in the case of CBN-grinding, there is always a latent risk ofexplosion,

in the grinding operation the amount of cooling/lubricating agent usedis substantially greater than in the case of cutting machiningprocedures as the cooling/lubricating agent is additionally employed inorder to remove the grinding dust and swarf from the surface of thegrinding wheel again, by jetting the cooling/lubricating agent on tosame under high pressure,

nonetheless the danger of the workpiece suffering from overheating isvery high,

the machining pressures acting on the workpiece are higher than in thecase of cutting machining, and

a microscopic surface structure is produced, in which the grainboundaries which are torn open by the grinding grain are smeared closedagain by the subsequent grinding grains, with removed workpiecematerial, that is to say this is a surface structure with relatively fewsteep peaks, but with more or less flat, bent-over peaks which partlyoverlap in scale-like relationship.

III. SUMMARY OF THE INVENTION

a) Technical Object

Therefore the object of the invention is to simplify the removal ofmaterial when machining bearing locations on a crankshaft.

b) Attainment of the Object

That object is attained by the characterising features of claim 1.Advantageous embodiments are set forth in the appendant claims.

By virtue of the grinding machining operation being omitted, themachining sequence is reduced from three to only two machiningprocedures which are in principle different. This eliminates not onlyall disposal problems in regard to the grinding slurry or swarf, butalso the quite considerable capital investment costs for grindingmachines, the costs involved in tool consumption and not least therequired stock of workpieces, which is increased due to the grindingoperation, by virtue of prolonged turn-around times for the workpieces.Disposal of the cuttings or swarf from the cutting machining operationdoes not give rise to any problems as either cutting is effected dry(high-speed milling) or separation of oil and cuttings or swarf isentirely possible by virtue of the much lower specific surface area ofthe cuttings or swarf in relation to grinding dust.

So that, in the procedure for removing material, the finishing operationcan directly follow the cutting machining operation with a given cuttingedge, hereinafter referred to for the sake of brevity as cuttingmachining, the degree of the admissible deviation of the actual valuesfrom the reference or target values, as occur after the cuttingmachining operation, must be so established that, in the totality of themachining procedures (cutting machining+finishing), the complication andexpenditure involved must be technically as low as possible, with at thesame time an overall machining time that is as short as possible.

In that respect it is not sufficient in the cutting machining operationto strive for reference or target dimensions which come as close aspossible to the final dimensions after the finishing operation, so thatthe oversizes which are to be dealt with by finishing and thus byrelatively slow removal of material can remain as small as possible.

It must be taken into account that the stroke deviation and the angledeviation of the big-end bearing journals can no longer or can be onlyvery slightly compensated by the finishing operation.

It must also be borne in mind that in the finishing operation in anycase firstly the amounts of material that can be removed (reduction indiameter) are very small, that is to say up to about 200 μm can beachieved at economically viable expense and in addition secondly thefinishing operation primarily provides for an increase in the percentagecontact area, more specifically by a reduction in roughness, with theaim of achieving a percentage contact area of about 95%. A percentagecontact area of 100% is unwanted as then there would no longer be anydepressions which are necessary in order to maintain a film of lubricantat the bearing.

In regard to the microscopic surface structure, the cutting machiningoperation gives a surface in which the grain boundaries are partiallytorn open by virtue of the cutting edge pulling the grains apart as itcuts through the material. As a result the surface has a relativelylarge number of pointed raised portions, interrupted by valleys in theform of opened grain boundaries. A surface structure of that kind isconducive to the finishing operation by virtue of the fact that the manypointed raised portions not only facilitate the removal of material bythe finishing procedure, but at the same time they also delay cloggingof the finishing belt and the like member which is used in the finishingoperation, insofar as the pointed raised portions of the workpiecesurface provide that the material which is already deposited in thefinishing belt is partially torn out of same again.

The limitations of this procedure are already encountered in theoperation of reducing roundness deviations by finishing, insofar as theamount of time involved or the roundness deviations which can be dealtwith depend not only on the absolute value of the roundness deviation tobe equalised or levelled, but also the configuration thereof:

If the non-roundness is such that there are only a few (for example 2-7)troughs and raised portions, distributed over the periphery (thusconstituting long-wave non-roundness), then, with the same absolutevalue in terms of non-roundness, for equalisation by means of finishing,a substantially greater amount of time is required or, under somecircumstances, it is not possible to provide for complete equalisation,in comparison with short-wave non-roundness involving at least 10 andpreferably even about 30 or more troughs per periphery of the bearinglocation, with the same absolute value in respect of non-roundness.

It is also to be borne in mind that in the finishing operation at thesame time on the one hand the degree of roughness is reduced and thusthe percentage contact area is improved, while on the other hand theexisting non-roundness is equalised or levelled. Those two effects canscarcely be decoupled from each other, or can be decoupled only to avery limited extent. If therefore, starting from an initial roughness,the desired roughness is achieved in the finishing operation after agiven period of time, the finishing procedure is stopped as a givenpercentage contact area should not be exceeded. The equalisation orlevelling effect in respect of the roundness deviation, which isachieved in that condition, is then accepted as a final result, andcannot be advanced separately any further.

Accordingly, if the procedure is commenced from a given initialcondition in respect of those two parameters from the finishingoperation, roughness and percentage contact area cannot be machinedindependently of each other to afford desired final values.

For the direct succession of the finishing operation after the cuttingmachining operation therefore what is recommended is in particularspecific coupling of the input parameters in regard to the finishingoperation and therewith the output parameters in regard to the cuttingmachining operation, in respect on the one hand of the absolute valueand degree of roundness deviation and on the other hand in respect ofmicroscopic roughness and the percentage contact area which applies inthat case.

In the cutting machining procedure using turning methods and also rotarybroaching methods and possibly also when using slow milling methods,long-wave roundness deviations rather occur by virtue of long-waveoscillations in the machine structure, the tools and the workpiecesinvolved in cutting machining. In contrast more especially high-speedexternal milling gives rise to short-wave roundness deviations.Therefore, the use of high-speed external milling in which an externalround milling tool of a diameter of about 700 cm, which is very large indiameter in comparison with the crankshaft, rotates at a cutting speedof between 150 and 1000 m/min beside the relatively slowly rotatingworkpiece about an axis which is parallel with respect to the crankshaftmeans that it is possible to achieve roundness deviations with manyraised portions along a circumference of a bearing.

Rotary milling, in particular if it is implemented with high cuttingspeeds, also rather tends to involve short-wave oscillations and thusshort-wave roundness deviations. For, rotary milling involves millingwith a kind of end-milling cutter which is arranged in paralleldisplaced relationship with respect to the radial direction of theperipheral surface, which is to be machined, of the bearing location,insofar as the peripheral surface is machined by means of the preferablyone or some cutting edges arranged on the end of the end-milling cutter.In that case, in particular machining with only one single cutting edgehas been found to be advantageous if in that case operation isimplemented with very high speeds of milling cutter rotation and theworkpiece rotates comparatively slowly. If in that case theabove-mentioned mirror surfaces or thrust surfaces of a bearing locationare also to be subjected to machining, the end-milling cutter is alsoprovided with one or more cutting edges on its peripheral surface.

It must also be taken into account that, in the finishing operation, thegrinding means which bears against the workpiece, for example afinishing belt, is generally not changed during the procedure. Thegrinding means therefore becomes increasingly clogged at its surfaceduring the finishing operation and the amount of material removed perunit of time progressively decreases.

How fast the grinding means begins to suffer from clogging in particularat the beginning of the finishing operation depends not only on theinitial roughness of the surface but also on the percentage contact areathereof:

The lower the percentage contact area with a given level of roughness—atthe beginning of the finishing operation—, that is to say the morepointed the microscopic surface structure is with correspondinglysteeper flanks, then all the more readily can the material particleswhich have been removed from the surface and deposited in the finishingbelt be removed from the finishing belt or the finishing stone orcomparable finishing means at the beginning, when dealing with such asurface structure. With increasing equalisation or levelling of themicroscopic surface the surface of the grinding means also becomesprogressively more quickly clogged in the finishing operation.

This means that, with the same degree of roughness, a percentage contactarea which is low at the beginning of the finishing operation isadvantageous for high initial removal of material and thus a finishingoperation which is short in time.

This also means that the degrees of roughness which can be handled bythe finishing operation increase, in inverse proportion to thepercentage contact area involved with those greater levels of roughness.

With the previous grinding operation, the roughness of the surface wasadmittedly reduced in comparison with preliminary cutting machining, butin that respect in the same way the percentage contact area was eitherkept constant or even increased as the preliminary cutting machiningoperation left behind a microscopic surface structure which involves alow percentage contact area as machining with the given cutting edge, inthe regions near the surface, means that in part the grain boundaries inthe metal structure are torn open, extending radially from the outsideinwardly.

In that way it is possible to finish in an economic fashion directlyafter the cutting machining operation insofar as efficient removal ofmaterial in the finishing operation is promoted and assisted by thepreliminary machining procedure, insofar as on the one hand the choiceof the correct cutting machining procedure means that the roughnessachieved in that case has a low percentage contact area, and theroundness deviation achieved in that case is a roundness deviation whichis as short-wave as possible.

In that respect it must further be taken into consideration that, in theprevious grinding of bearing locations, the roundness deviationsresulting from the preliminary cutting machining were generally onlyreduced by the grinding operation in terms of their absolute value butnot in terms of their characteristic. Therefore, the grinding operationdid not result in long-wave roundness deviations becoming short-waveroundness deviations, but the number of troughs was either retained oreven reduced, with the consequence that a further improvement inroundness deviations by the finishing operation, considered as animprovement in result per unit of time, was made rather more difficultin the finishing procedure.

That means that a finishing operation directly after the cuttingmachining procedure is particularly economical when, after the cuttingmachining operation, the roundness deviations are less than 60 μm and inparticular less than 40 μm, the diameter deviation is less than 200 μm,in particular less than 150 μm and the roughness Ra is less than 10 μm,in particular less than 6 μm. In that respect, the aim to be sought isroundness deviations with a short-wave nature of at least 30 waves percircumference, which applies in regard to bearing diameters of about 50mm, but which, with rising or falling bearing diameters, should onlychange in a sub-proportional fashion, that is to say for example a 100%change in circumference produces only about a 30% change in the numberof waves.

Furthermore, in that respect, the aim to be sought is a rather lowerpercentage contact area in respect of the roughness achieved after thecutting procedure, than is obtained after the grinding procedure.

In the case of big-end bearings in addition the angle deviation afterthe cutting machining operation should be less than 0.4°, in particularless than 0.2°, and the stroke deviation should be less than 0.40%, inparticular less than 0.20%, which corresponds to the tolerances to beobserved in regard to the crankshaft when ready for use, as thoseparameters can no longer be changed in an economically viable manner bythe finishing operation.

A suitable form of the cutting machining procedure is therefore externalmilling or rotary milling, in particular in the form of high-speedmilling, in consideration of the above-described interrelationships.

Particularly when dealing with heavy workpieces, it has been found thata combination of the deviations between reference or target values andactual values of the relevant parameters, such deviations occurringafter the cutting machining operation and prior to the machiningoperation, wherein such combination is desirable in terms of directcoupling of cutting machining and finishing, can only be achieved if thecutting machining procedure is effected in a plurality of stages, inparticular in two stages (preliminary cutting and finishing cutting). Inthat respect, high-speed external milling or high-speed rotary millingis to be preferred both when dealing with big-end bearings and also whendealing with main bearings, for the second stage of the finishingcutting operation.

The first stage of the preliminary cutting operation when dealing withbig-end bearings will also be effected using external milling, inparticular using high-speed external milling, while when dealing withmain bearings this can also be effected by turning or rotary broachingor turning-rotary broaching.

If the cutting operation is implemented in two or even more stages, theoversizes which are to be dealt with in the finishing cutting operationrange in the optimum fashion between 0.2 and 0.5 mm in order further toimprove in particular roundness and diameter deviation by virtue of theremoval of very thin cuttings or swarf and in order to achieve aroughness which remains as equal as possible in the course of eachindividual step in the high-speed milling procedure, with the percentagecontact area remaining uniformly low, from a microscopic point of view.

On the other hand, the use of belt-type finishers is recommended for thefinishing operation, in which case the grinding belts are pressed bymeans of contact pressure shell members against the rotating bearinglocation and at the same time a relative oscillation is produced betweenthe grinding belt and the workpiece, in the longitudinal direction. Inthat case, the contact pressure shell members should embrace theworkpiece by at least 120° in each case, preferably by up to 180°.

If in addition during the finishing process the instantaneous actualdimension is checked and tested and the procedure is controlled inregard to speed of rotation and contact pressure, the diameter deviationcan be reduced in a particularly good fashion without excessive heatbeing introduced into the workpiece due to high levels of frictionalforce, and without thereby causing distortion of the workpiece. Asuitable finishing procedure is described for example in U.S. Pat. No4,682,444. The finishing operation is ideally effected dry, that is tosay without the addition of cooling/lubricating agents to the machininglocation, although this cannot always be achieved.

c) Embodiments

The above-mentioned parameters are described in greater detailhereinafter with reference to the drawings in which:

FIG. 1 is a view in cross-section through a big-end bearing journal,

FIG. 2 is a view in cross-section through a bearing journal in general,and

FIG. 3 is a view in cross-section of a contour whose concentricity is tobe determined.

FIG. 1 is a view in cross-section through a big-end bearing journal forexample after the cutting operation and prior to the machiningoperation, wherein solid thick lines denote the actual contour thereof,the broken line denotes the reference or target contour thereof afterthe cutting operation and the dotted or short-dashed line shows thereference or target contour thereof after the finishing operation, thatis to say the final contour thereof.

In this respect the reference or target contours are exactly circularcontours around a target or reference center which has a given referenceor target stroke, that is to say a radial spacing from the main bearingcenter of the crankshaft.

The actual contour of the big-end bearing journal is in comparisonnon-round. The non-roundnesses are shown in drastically exaggerated formin the Figure.

In this case, shown in the top right quadrant is a short-wave roundnessdeviation with a large number of wave crests and troughs per anglesegment, while shown over the remainder of the circumference is along-wave roundness deviation with few waves and troughs.

In regard to the individual parameters which are utilised for assessingthe quality of a rotationally symmetrical surface and in particular thebearing locations of crankshafts, a distinction is to be drawn betweenparameters which are related to the reference or target center of therespective bearing journal, and parameters which are determinedindependently of the reference or target center.

Macroscopic parameters which are not related to a given reference ortarget center are roundness and diameter deviation.

Regarding the parameters more specifically:

Roundness:

So-called roundness, that is to say in actual fact the deviation fromthe ideal circular reference or target contour, is determined inaccordance with ISO 1101, point 3.8 thereof, in that the actual contouris fitted in between two mutually concentric circles, the inner circleKi and the outer circle Ka, as tightly as possible. The two circles mustbe concentric only relative to each other but not relative to thereference or target center. The radial spacing, that is to say half thediameter difference, of those two circles Ki and Ka, is identified asthe roundness.

As the circles Ki and Ka must extend in mutually concentricrelationship, their center point, the roundness center, is not alwaysthe same as the actual center which for example is ascertained as thecenter of gravity of the actual contour when considered as a surfacearea.

Roundness is thus virtually the heightwise spacing between the highestwave crest and the deepest wave trough of the developed actual contour.

Diameter Deviation:

Here what is important first of all is whether the reference or targetdiameter after the respective machining step or the final diameter, thatis to say the reference or target diameter after the finishing step, isadopted as the reference or target diameter. In general the maximumlimits in respect of diameter deviation, which are contained in thespecification catalogue for a machining procedure or a machiningmachine, relate to the reference or target value after that specificmachining step.

FIG. 2 shows both the reference or target contour after the cuttingoperation and also the reference or target contour after the finishingoperation, that is to say the final contour, in addition to the actualcontour.

The diameter deviation can be ascertained in at least two fundamentallydifferent ways:

The procedure, which was earlier the conventional one, for checkingdiameter provided that the machined contour was introduced eitheraxially into sleeve-shaped or socket-shaped spaces or gauges of knowninternal diameter or radially into fork-shaped spaces or gauges in whichthe free spacing between the ends of the fork was known. Spaces orgauges of that kind were available in small dimensional steps and, ifthe test piece could still just be introduced into the one space orgauge but could no longer be introduced into the next smaller one, thatmeant that the diameter dimension of the test piece was known as beingbetween the dimensions of the two gauge sizes.

The measurement method using a sleeve-like gauge and—when involvingmultiple implementation in different cross-sectional planes—also themethod using a fork-shaped gauge, therefore always determined themaximum actual diameter.

Nowadays, having regard to high demands in terms of quality, test piecesare generally measured with equipment referred to as measuring machines,and the desired values are determined by the measuring machine. In thecase of a diameter deviation the measuring machine, with its sensingdevice, measures the peripheral contour of the desired bearing journalin a given plane or also in a plurality of planes. As all individualdiameters, in each desired angular position, are known therefrom, it ispossible to calculate therefrom for example a mean, averaged actualdiameter.

The diameter deviation A lies in the difference between the maximumactual diameter and the reference diameter, as shown in FIG. 1.Accordingly, roundness or a deviation from ideal roundness is entirelyradially within the maximum actual diameter.

If the diameter deviation is determined from the averaged actualdiameter and the reference or target diameter, that affords lower valuesin respect of the diameter deviation as the roundness is partly insideand partly outside the averaged actual diameter.

In both cases however it is to be noted that, after the machining stepwhich is being considered at present, for example the cutting operation,as shown in FIG. 2, the reference or target contour admittedly does notnecessarily have to be completely within the actual contour, but thefinal contour does indeed have to.

In contrast the final contour can certainly be at least partly outsidethe inner circle Ki which is used to determine roundness.

When determining the diameter deviation B from the averaged actualdiameter and the reference or target diameter, for dimensional accuracyof the test piece it is additionally necessary to take account of theproportion by which the roundness extends radially outwardly from thataveraged actual diameter. It must also be known whether the averagedactual diameter deviates positively or negatively from the reference ortarget diameter after the respective machining step, that is to say forexample after the cutting operation, as the averaged actual diameter, asshown in FIG. 1, can certainly also be smaller than the reference ortarget contour after the cutting operation without the test piece havingto be deemed to be waste. As long as the averaged actual diameter isstill larger than the final contour, the final contour can be attainedby the subsequent machining step, for example the finishing operation.

For the sake of enhanced clarity of the drawing, the circles Ki and Ka(which admittedly must extend concentrically relative to each other butnot relative to a given center and which are to be at the minimumspacing from each other) for determining roundness, together with theaveraged actual diameter and the maximum actual diameter, are shown inFIG. 1 only in the form of portions.

A macroscopic parameter which is of significance both for big-endbearing journals and also for main bearing journals and whichnonetheless is based on the relationship relative to the reference ortarget center is:

Concentricity (FIG. 3):

This is determined by applying minimally spaced circles Di and Da to theactual contour of the test piece, at the inside and the outside. Incomparison with determining roundness however, those two circles Di andDa are arranged not only concentric relative to each other but alsoconcentrically relative to the reference or target center.

The annular region between the circles Di and Da therefore representsthat annular region which the actual contour of the for example mainbearing surface would sweep, upon rotation of the crankshaft.

In particular for the illustrated big-end bearing journals, as shown inFIG. 1, the deviation of the actual center from the reference or targetcenter is an aspect of crucial significance which is no longerinfluenced by the finishing operation or which is only slightlyinfluenced thereby.

While the stroke deviation, that is to say the deviation of the actualstroke from the target or reference stroke, undesirably alters thereference or target compression of the reciprocating piston engine inwhich the crankshaft is later installed, the angular deviation, that isto say the deviation of the actual angular position of a big-end bearingjournal with respect to the overall crankshaft from the reference ortarget angular position thereof influences the angular position of thedead center point of that big-end bearing location in the reciprocatingpiston engine, that is to say in the case of an internal combustionengine inter alia the ignition firing point, the optimum valve openingand closing times and so forth.

Stroke Deviation and Angle Deviation

After the cutting operation therefore these must already be within thetolerances for the final dimensions.

In comparison with the previous macroscopic considerations, the bottomright part of the Figure is a view on an enlarged scale of themicroscopic surface structure.

Roughness:

Denotes subjectively ascertained, microscopic surface configuration.

Roughness R_(a):

In this respect, this denotes the arithmetic mean value, determined inaccordance with DIN 4768, of the absolute values of all profileordinates of the microscopic surface profile, usually ascertained withinan overall measuring section and after filtering out of coarsedeviations in respect of shape and relatively coarse components such asroundness, that is to say waviness of the surface.

Nonetheless this frequently employed parameter scarcely permitsconclusions to be drawn in regard to the height of the roughnessprofile. Therefore, for better illustration of the situation, referenceis frequently made to:

Roughness Depth R_(z):

(also in accordance with DIN 4768). This parameter represents thespacing between the highest raised portion and the lowest or deepestdepression in a microscopic surface structure within a defined testsection, wherein the value ascertained in that way is averaged over fivetest sections for determining the value of R_(z) in order not toovervalue in the calculation procedure freak values from the surfaceprofile, that is to say extremely high points and extremely deeptroughs.

Percentage Contact Area

When viewed in a development, this is that proportion of the surfacewhich, when levelling off the microscopic roughness to a given residualheight, occurs as a continuous surface proportion.

In a practical context, the percentage contact area is ascertained by aprocedure which comprises pressing against the surface to be determined,a counterpart surface of ideal shape, that is to say when dealing withflat surfaces, an ideally flat surface or, in the present case, whendealing with external round surface, a concave counterpart surface whichideally corresponds to a circular arc, under a given nominal loading,for example 0.1 N/mm². By virtue of that nominal loading, themicroscopic raised portions of the profile which without a loading wouldonly bear against the counterpart surface with their tips and thus witha surface proportion of tending closely towards 0 are pressed somewhatflat so that the contacting surface proportion rises with respect to thetotal surface area and in practice can be satisfactorily ascertained bydyeing or tinting and so forth. At the given nominal loading mentioned,the percentage contact area is between 20% and 40% at a transfer betweencutting machining and finishing operations. This same percentage contactarea is less than 50% of the percentage contact area occurring after thefinishing operation.

In this case also, a given reference surface area is taken as the basicstarting point in the operation of determining the percentage contactarea. However, no standard in accordance with DIN exists for determiningthe percentage contact area, but only a VDI-Guideline in that respect,more specifically VDI/VDE 2603.

The percentage contact area therefore correlates with that surfaceproportion which, in subsequent use of the bearing journal, can actuallybe supported against the bearing shell which is disposed in externalopposite relationship therewith, although in that practical use dynamicforces, the radially acting forces, additionally act in that bearingcombination, and thus still further increase the load-bearing surfacewith respect to the percentage contact area ascertained by a definedstatic loading.

The residual microscopic depressions remaining between that continuoussurface proportion serve for example to accommodate lubricant, toreceive microscopic wear or abrasion or molecular flow movements of thematerial and so forth, and for that reason a percentage contact area of100% is not wanted in connection with mechanical bearing locations suchas those of a crankshaft, but a maximum of about 95%.

What is claimed is:
 1. A method of finishing machining a bearing location of a crankshaft comprising: performing an original shaping operation, effecting removal of material only by a cutting machining operation with a given cutting edge and a subsequent finishing operation, and effecting a transfer from the cutting machining operation to the finishing operation when: a) a roundness deviation of the bearing location is less than 60 μm, and b) a diameter deviation of the bearing location is less than 200 μm.
 2. A method as set forth in claim 1 and further comprising effecting no hardening of a bearing surface between the cutting machining operation and the finishing operation.
 3. A method as set forth in claim 1 and further comprising forming said crankshaft at least partly from cast iron.
 4. A method as set forth in claim 1 and further comprising providing the bearing location with a percentage contact area, when pressed against an ideally shaped counterpart surface with a pressure of 0.10 N/μm², of between 20% and 40% at the transfer from the cutting machining operation to the finishing operation.
 5. A method as set forth in claim 1 and further comprising providing the bearing location with a percentage contact area at the transfer from the cutting machining operation to the finishing operation which is less than 50% of a percentage contact area occurring after the finishing operation.
 6. A method as set forth in claim 1 and further comprising providing an angle deviation of a bearing location position from a reference position thereof of less than 0.40° or a stroke deviation between an actual stroke and a reference stroke of less than 0.40% of the reference stroke after the cutting machining operation.
 7. A method as set forth in claim 1 and further comprising effecting said cutting machining operation by preliminary cutting and finishing cutting.
 8. A method as set forth in claim 7 and further comprising effecting said finishing cutting by external or rotary high-speed milling.
 9. A method as set forth in claim 7 and further comprising effecting said preliminary cutting by rotary broaching or turning-rotary broaching.
 10. A method as set forth in claim 7 and further comprising effecting said preliminary cutting by external milling or rotary milling in the form of high-speed milling.
 11. A method as set forth in claim 1 and further comprising effecting the finishing operation by a grinding means which is applied dry against a surface of said bearing location and which rotates and oscillates relative to said surface of said bearing location.
 12. A method as set forth in claim 11 and further comprising pressing a finishing belt against said bearing location with a defined force by contact pressure shell members which embrace said bearing location over at least 120° while effecting the finishing operation.
 13. A method as set forth in claim 1 and further comprising effecting said transfer only when a roughness of the bearing location is less than 10 μm, and the bearing location has at least 10 wave troughs per circumference. 